DIN EN 61800-5-1 provides the framework for safe and reliable temperature measurement in motors within electrical drive systems. It ensures that temperature monitoring meets the required safety standards and thus contributes to the protection of the motor and the entire drive system.

Until now, engine manufacturers have mainly used KTY-84-130 sensors. However, these are now rarely used, as the standard requirements for temperature measurement in power drive systems have increased and some manufacturers have discontinued the KTY versions.

Currently, Pt1000 temperature sensors are also predominantly installed in the motors. The design of such a temperature sensor with a Pt1000 has been retained in the same way as the KTY. This means that heat-shrinkable tubing is often applied to the measuring point as an insulating material, either once or several times. However, this design only partially fulfills the increased demands on the required insulation system with regard to the more stringent standard requirements: The heat-shrink tubing is made of polyvinylidene fluoride (PVDF), which belongs to the group of per- and polyfluorinated alkyl substances (PFAS). These substances have been criticized for their persistence in the environment.

Central challenges

Example of a wound stator with rotor insert © Jumo

PGT Thermprozesstechnik, a wholly owned subsidiary of Jumo, has developed new types of temperature sensors for the motor industry for sensing hotspot temperatures. The task was to ensure that the temperature sensors also meet the new standard requirements for dielectric strength and the insulation system in the power drives.

The central requirements here were:

Oval Pt1000 sensor head design 3 mm/3.4 mm © Jumo

• Use of a Pt1000 platinum measuring resistor in accordance with IEC 60751
• Ensuring withstand voltage resistance as permanent electrical insulation between sensor and motor winding (standard requirement DIN EN 61800-5-1)
• Ensuring surge voltage safety to withstand short-term voltage peaks or transient overvoltages (standard requirement DIN EN 61800-5-1)
• Achieving the normative partial discharge resistance to ensure the safety, reliability and durability of the insulation system of the new temperature sensors in the motor
• Development of a measurement method for the reproducible measurement of partial discharges in the sensor, on the connecting wires and in the manufacturer's motor system
• Specification of a temperature-resistant stranded wire with a high dielectric strength and an outer diameter of no more than 1 mm
• Selection of all components under the aspect of harmonization according to UL 61800-5-1 and CSA 22.2 No. 274
• Development of a functional, recyclable and thermally modified thermoplastic material
• Development of an injection molding tool concept and an injection molding process that ensures that the plastic melt does not thermally or mechanically destroy the measuring element during injection molding
• Scaling of the mold concept from a prototype single-cavity mold to a multi-cavity mold (4-cavity)
• Optimization of the manageability of insert assemblies through mechanical decoupling with the help of developed fixing aids
• Validation of connection techniques with regard to the connection of stranded wires to the sensor connection to ensure tightly toleranced component heights and contacting positions so that the sensor's insulation system complies with the standard
• Definition of the clearances and creepage distances in the sensor head to ensure the dielectric strengths according to the standard
• Development of a multiple test device for measuring the sensor signal at the cold end of the sensor, after the injection molding process, for all common cable ends

The result from PGT includes a thermoplastic polymer specially developed for this product, which injection molds the sensor head using a likewise newly developed injection molding process. PVDF shrink tubing is therefore no longer necessary. Due to the high quantities on the market, the injection molding process is also suitable for producing very high quantities with a reproducible measuring element position in the sensor head to maintain the insulation system.

The sensors are currently manufactured in a PGT 4-cavity injection mold.

This process has enabled the following new product features to be realized:

• UL-approved components
• Freely selectable design for the sensor head (standard diameter 3 to 4 mm, length 16 to 18 mm)
• Connection wire with 1 mm diameter and up to 13 kV dielectric strength
• Thermally conductive sensor head made of thermoplastic (2 to 6 W/mK)
• Shock and vibration resistance
• Designable withstand voltage strength in the range from 4 to 10 kV in accordance with DIN EN 61800-5-1
• Surge voltage resistance in accordance with DIN EN 61800-5-1
• Temperature approval for all insulation systems A, E, B, F, H, N (+105 to +220 °C)
• Safe partial discharge resistance of the sensor assembly <10 pC at 1.5 × UPD (15 sec.)
• Temperature range -40 to +240 °C
• Fast response time/response time of the sensor head in the range of a few seconds (t90)
• Use of a Pt1000 measuring resistor in SMD design or as a wired version (Jumo)

Michael Klose is a press spokesman for Jumo in Fulda. © Jumo

This means that the motor market now has access to temperature sensors that meet the current requirements of the DIN EN 61800-5-1 standard as well as the increased requirements of the future standard. The sensors are currently being manufactured in large quantities for industrial motors and vehicle motors. They are used to reliably detect temperature increases in or on motor winding stacks. The measurement is carried out directly on the live conductors.

The newly developed technology of plastic encapsulation of sensitive measuring elements is also used for cross-industry solutions. In addition to the processing of analog measuring resistors, digital temperature sensors can also be shaped in this way.